1. Field of the Invention
The invention relates to the field of earth boring tools, and more particularly to rotary jars used in connection with coring tools to free jammed inner core barrels.
2. Description of the Prior Art
Rotary jars are well known to the art and typically include a hammer and anvil in a downhole tool used for transmitting an impulsive force to a fishing tool such as shown by Foster "Oil Well Jar", U.S. Pat. No. 2,153,883, or to a core barrel such as shown by Burt "Core Barrel", U.S. Pat. No. 2,005,989. Typically, such rotary jars use a hammer of cylindrical shape which is rotated about the longitudinal axis of the downhole tool. The hammer has a plurality of ramped surfaces on its butt end which mate a similarly shaped anvil. As the hammer rotates the ramped surfaces of the hammer and anvil ride up against each other to a point of maximum separation and then suddenly jam back together into a mating configuration when the ramped teeth of the hammer and anvil are once again aligned. Examples of such rotary hammers are shown in McLean et al. "Rotary Jar", U.S. Pat. No. 1,535,935, Erwin "Rotary Jar", U.S. Pat. No. 2,153,882, Foster "Oil Well Jar", U.S. Pat. No. 2,153,883 and Fleming "Jar for Fishing and Drilling Tools", U.S. Pat. No. 1,653,093.
However, typically such rotary jars are connected to the fishing tool or coring barrel at a point well above the tool or outside the barrel, such as shown in Burt, supra, as best shown in FIG. 2.
Moreover, prior art jars either continually operate or are selectively activated by some type of mechanism from the well platform. As a result the jar is either operating when not required or not operating for some period when required. In the latter case, only later, after the jam becomes obvious, is the jar activated. Often the jam is then much more severe and is sometimes impossible to unjam. In the former case, continual jarring is not only unnecessary, but may disturb the unjammed core, degrade drilling performance or prematurely wear or cause equipment to fail. Furthermore, prior art jars generally respond with a single magnitude of jarring force regardless of the severity of the jam.
Therefore, what is needed is a rotary jar in combination with a coring tool: wherein an impulsive force is applied to the inner barrel of the coring tool automatically and as soon as a jam occurs but not before; wherein the jar automatically ceases operation when no longer required; wherein a multilevel unjamming force is produced by the jar; and wherein the rotary jar mechanism is contained entirely within the inner barrel to facilitate use of the coring tool during normal, unjammed coring operation.